Merely enabling compile-testing should not enable additional code.
To fix this, restrict the automatic enabling of BCM4908_ENET to
ARCH_BCM4908.
Fixes: 4feffeadbcb2e5b1 ("net: broadcom: bcm4908enet: add BCM4908
controller driver")
Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>